Methods for screening compound libraries

ABSTRACT

Disclosed are methods for screening compound libraries using frontal chromatography in combination with mass spectrometry to identify and rank those members of the library that bind to a target receptor. Methods are also disclosed which permit a compound library to be rapidly screened to determine if any member of the library has an affinity for the target receptor as measured by a pre-selected indicator compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 09/390,694 filed Dec. 28, 1998 and now abandoned, which in turn is a continuation of U.S. Ser. No. 09/070,131 filed Apr. 29, 1998 now abandoned which claims the benefit of U.S. Provisional Application Ser. No. 60/079,622 filed Mar. 27, 1998 and now expired.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for screening compound libraries, such as compound libraries generated using combinatorial chemistry techniques. The methods of this invention employ frontal chromatography in combination with mass spectrometry to screen a library of compounds to identify and rank those members of the library that bind to a target receptor. The methods of this invention also permit a compound library to be rapidly screened to determine if one or more members of the library have an affinity for a target receptor as measured by a pre-selected indicator compound.

2. References

The following publications, patents and patent applications are cited in this application as superscript numbers:

¹ K. S. Lam, Anti-Cancer Drug Des. 1997, 12, 145-167.

² P. M. Sweetnam et al., In Burger's Medicinal Chemistry and Drug Discovery; M. E. Wolff, Ed.; John Wiley & Sons: New York, 1995; pp 697-731.

³ R. H. Griffey et al., In Proceedings of the 45^(th) ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, Calif., Jun. 1-5, 1997; p. 400.

⁴ L. Fang et al., In Proceedings of the 45^(th) ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, Calif., June 1-5, 1997; p. 401.

⁵ Y. -H. Chu et al., J. Am. Chem. Soc. 1996, 118, 7827-7835.

⁶ Y. -Z. Zhao et al., J. Med. Chem. 1997, 40, 4006-4012.

⁷ Y. F. Hsieh et al., J. Mol. Div. 1996, 2, 189-196.

⁸ R. W. Nelson et al., Anal. Chem. 1995, 67, 1153-1158.

⁹ D. C. Schriemer and L. Li, Anal. Chem. 1996, 68, 3382-3387.

¹⁰ PCT/US97/07964 (International Publication No. WO 97/43641), published Nov. 20, 1997, entitled “Molecular Diversity Screening Device and Method.”

¹¹ R. Wieboldt et al., Anal. Chem. 1997, 69, 1683-1691.

¹² R. B. van Breemen et al., Anal. Chem. 1997, 69, 2159-2164.

¹³ M. L. Nedved et al., Anal. Chem. 1996, 68, 4228-4236.

¹⁴ PCT/US95/03355 (International Publication No. WO 95/25737), published Sep. 28, 1995, entitled “Method for Identifying Members of Combinatorial Libraries.”

¹⁵ PCT/EP97/02215 (International Publication No. WO 97/43301), published Nov. 20, 1997, entitled “Identification of Members of Combinatorial Libraries By Mass Spectrometry.”

All of the above publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety.

3. State of the Art

In recent years, a large number of combinatorial chemistry techniques have been developed which permit vast libraries of diverse chemical compounds to be rapidly synthesized.¹ In combinatorial chemistry, a series of chemical reactions is typically conducted employing a plurality of reagents at each step to generate a library of compounds. Such techniques have the potential to greatly accelerate the discovery of new compounds having biologically useful properties by providing large collections of diverse chemical compounds for biological screening.

This ability to rapidly generate large collections of compounds using combinatorial chemistry techniques has created a need for new methods of screening compound libraries. The traditional approach of screening each compound individually in an assay to identify those compounds having the desired biological activity is no longer practical due to time and resource constraints. Thus, a need exists for new methods which permit the rapid screening of compound libraries.

In this regard, various methods for screening compound libraries have been reported. Typically, these screening methods involve the use of target receptors which have been labeled with fluorescent or other reporter groups.² In these methods, the compound library, typically bound to a resin bead, is exposed to the labeled target receptor and those members binding to the labeled target receptor are identified and physically separated. The particular ligand binding to the target receptor is then identified. In many of these techniques, elaborate procedures are required to keep track of individual members of the library. For example, coded tags are often added during the synthesis of the combinatorial library to allow the structure of the individual members to be subsequently determined. Alternatively, combinatorial libraries can be prepared in an array and the individual members of the library subsequently identified by their location in the array. While such methods can be effective, the need to keep track of individual members of the library during their synthesis and screening is quite cumbersome and often limits the type of synthetic procedures that can be employed. Additionally, many of these techniques require that the synthetic procedures be conducted on a solid phase, thus further limiting the synthetic procedures and reagents that can be used.

As an alternative, mass spectrometry is emerging as an important tool for the interrogation of combinatorial libraries. To date, mass spectrometry has been used to assess library quality^(3,4) and, when coupled with molecular recognition technologies, has allowed for some success in the isolation and characterization of active library compounds.⁵⁻¹⁵ Typically, when screening compound libraries for biologically active members, mass spectrometry is used in combination with a “capture and release” methodology. In this methodology, compound mixtures are presented to the target receptor, which is often immobilized on a solid support, and the resulting ligand-receptor complexes are separated from the unbound members of the library. After separation, the ligand-receptor complexes are typically denatured, for example, with a solvent and the solvent mixture containing the previously bound ligands is presented to the mass spectrometer to permit identification of the high affinity ligands.

For example, ultrafiltration has been used in combination with electrospray mass spectrometry to screen combinatorial libraries.¹⁰⁻¹² In this method, ligands present in a compound library are allowed to bind to a receptor and the resulting ligand-receptor complexes are purified by ultrafiltration. The ligand-receptor complexes are then dissociated using a solvent, such as methanol, and the previously bound ligands are detected by an electrospray mass spectrometer.

Affinity capillary electrophoresis (ACE) has also been coupled with mass spectrometry to screen combinatorial libraries.⁵ In this procedure, ACE is used to separate ligand-receptor complexes from unbound ligands and mass spectrometry is used to identify the high affinity ligands.

Similarly, compound libraries have been screened using affinity chromatography in combination with mass spectrometry. For example, WO 97/43301 describes a method for characterizing the members of a combinatorial library, which method utilizes affinity selection in combination with mass spectrometry. Specifically, the members of the library are brought into contact with a domain of interest to allow for binding, i.e., the formation of a complex. After binding, the complex is separated from the unbound members of the library, typically by washing the unbound members from the column containing the complexes. The complexes are then treated to elute the bound library components and the eluted components are analyzed by mass spectrometry. The elution methods described include the use of displacers, chaotrope agents, pH elution, salt gradients, temperature gradients, organic solvents, selective denaturants and detergents. Using such methods, the weakly bound members of the library are purportedly eluted first and analyzed by mass spectrometry, followed by the elution of the more strongly bound members.

There are several disadvantages associated with the “capture and release” methods for screening compound libraries that have been previously reported. First, the procedure used to “release” the bound ligands from the ligand-receptor complexes may alter the binding profile for the various bound ligands, resulting in a false indication of binding strength. For example, using a pH gradient to release the bound members of the library may change the electronic character of the binding site on the receptor causing ligands which are strongly bound under physiological conditions to be prematurely released. Thus, the characterization of binding strength for various ligands based on their relative time of release may be misleading if the release conditions are different from the binding conditions. Accordingly, these methods may not accurately identify the most active members of a compound library. Additionally, certain conditions used for compound release, such as pH gradients, may irreversibly denature the receptor thus preventing its subsequent use for screening compound libraries.

Additionally, when “capture and release” methods are employed, each bound ligand is typically released over a relatively short period of time resulting, for example, in an elution peak or “spike” for each ligand. Accordingly, the effluent produced using such methods is typically monitored continually, for example, by mass spectrometry so that any particular elution peak is not missed. Thus, the number of analyses that can be conducted using any particular mass spectrometer is limited. Accordingly, it would be desirable to develop methods for screening compound libraries that do not rely upon “capture and release” methodologies.

SUMMARY OF THE INVENTION

This invention is directed to methods for screening compound libraries. The compound libraries may be generated or obtained by any means including, by way of example, combinatorial chemistry techniques or from fermentation broths, plant extracts, cellular extracts and the like. The methods of this invention employ frontal chromatography (FC) in combination with mass spectrometry (MS) to screen the library of compounds to identify and rank those members of the library that bind to a target receptor.

In frontal chromatography, a target receptor is typically immobilized on a suitable solid support material and packed in a column. A mixture containing putative ligands is then continuously infused through the column. Ligands having an affinity for the target receptor bind to the column, but eventually the capacity of the column for each ligand is exceeded and the ligands elute or “break through” at their infusion concentration. Once a ligand begins eluting from the column, it is continually present in the effluent. Compounds having little or no affinity for the target receptor break through earlier in the effluent compared to ligands having a higher affinity for the receptor.

In the present invention, mass spectrometry (MS) is employed to continuously or intermittently monitor the FC effluent. Using MS, the identity and break through time for each ligand on the column can be determined. Thus, FC-MS allows the relative affinity of each member of the library for the target receptor to be determined relative to other members of the library under ligand-receptor binding conditions. Using the present methods, an accurate ranking of the relative affinity of each member of the compound library for the target receptor can be ascertained.

Accordingly, in one of its method aspects, the present invention is directed to a method for screening a compound library to determine the relative affinity of a plurality of putative ligands to a target receptor or a plurality of target receptors, which method comprises:

(a) providing a compound library comprising a plurality of putative ligands,

(b) applying the compound library to a column comprising a target receptor or a plurality of target receptors, each target receptor optionally bound to a solid phase support, under frontal chromatography conditions to provide an effluent;

(c) continuously or intermittently applying the effluent to a mass spectrometer to provide mass spectra of the constituent putative ligands present in the effluent; and

(d) evaluating the mass spectra to determine a break through time for the putative ligands.

In a preferred embodiment, the above method further comprises the step of:

(e) determining an affinity to the target receptor for a putative ligand in the compound library relative to another putative ligand in the library by comparing the break through time for the putative ligand to the break through time for the other putative ligand.

In another preferred embodiment, the above method further comprises the step of:

(f) determining a dissociation constant, K_(d), for a putative ligand in the compound library and the target receptor.

Preferably, the compound library employed in the above method comprises less than about 10,000 putative ligands, more preferably less than about 5,000 putative ligands. Still more preferably, the compound library comprises about 5 to about 100 putative ligands.

In one embodiment of this invention, the compound library preferably comprises putative ligands selected from the group consisting of carbohydrates, monosaccharides, oligosaccharides, polysaccharides, amino acids, peptides, oligopeptides, polypeptides, proteins, nucleosides, nucleotides, oligonucleotides, polynucleotides, lipids, retinoids, steroids, glycopeptides, glycoproteins, glycoproteins, proteoglycans, and synthetic analogs or derivatives thereof, and the like.

In another embodiment, the compound library comprises putative ligands selected from the group consisting of natural products.

In another embodiment, the compound library comprises putative ligands selected from the group consisting of synthetic small molecule organic compounds.

Preferably, each target receptor is independently selected from the group consisting of proteins, including recombinant proteins, glycoproteins, glycosaminoglycans, proteoglycans, integrins, enzymes, lectins, selectins, cell-adhesion molecules, toxins, bacterial pili, transport proteins, receptors involved in signal transduction or hormone-binding, hormones, antibodies, major histocompatability complexes, immunoglobulin superfamilies, cadherins, DNA or DNA fragments, RNA and RNA fragments, whole cells, cell fragments, tissues, bacteria, fungi, viruses, parasites, preons, and synthetic analogs or derivatives thereof.

Additionally, the target receptor is preferably bound to a solid phase support. More preferably, the target receptor is covalently bound to the solid phase support or bound via biotin-avidin or biotin-streptavidin binding.

Preferably, the solid phase support used in this invention is selected from the group consisting of polymeric (resin) beads, polymeric gels, glass beads, silica chips, silica capillaries, agarose, diatomaceous earths and pulp.

The column employed in the methods of this invention preferably contains from about 1 fmol to about 10 nmol of target receptor active sites; preferably, from about 1 pmol to about 10 nmol of target receptor active sites.

In the methods of this invention, the effluent from the column is preferably diluted with a supplemental diluent before analysis by mass spectrometry. Preferably, the supplement diluent comprises a major amount of an organic solvent and a minor amount of an aqueous buffer. The organic solvent is preferably selected from the group consisting of acetonitrile, methanol and isopropanol.

Preferably, the mass spectrometer employed in the methods of this invention is an electrospray mass spectrometer.

Additionally, since FC-MS does not require constant effluent monitoring, a plurality of FC-MS analyses can be conducted simultaneously using a single mass spectrometer to intermittently monitor each column. Furthermore, under FC conditions, since ligands are always present in the effluent once they break through the column, the intermittent monitoring of each column does not necessarily require monitoring the actual break through time for each ligand. Therefore, a plurality of FC-MS analyses can be conducted simultaneously using a single mass spectrometer to intermittently monitor each column.

Accordingly, in another of its method aspects, this invention provides a method for screening a plurality of compound libraries to determine the relative affinity of a plurality of putative ligands in each library to a target receptor, which method comprises:

(a) providing a plurality of compound libraries, each library comprising a plurality of putative ligands,

(b) applying each compound library to a separate column comprising a target receptor or a plurality of target receptors, each target receptor optionally bound to a solid phase support, under frontal chromatography conditions to provide an effluent from each column;

(c) intermittently applying the effluent from each column to a mass analyzer of a mass spectrometer to provide mass spectra of the constituent putative ligands present in the effluent; and

(d) evaluating the mass spectra to determine a break through time for the putative ligands in each compound library.

In a preferred embodiment, the above method further comprises the step of:

(e) determining an affinity to the target receptor for a putative ligand in a compound library relative to another putative ligand in the same library by comparing the break through time for the putative ligand to the break through time for the other putative ligand.

Preferably, from 2 to about 100 columns are used in the methods of this invention which employ a plurality of columns. More preferably, 2 to about 50 columns are employed and still more preferably, 2 to about 10 columns are employed.

In another embodiment, this invention provides a method for screening a compound library to determine if putative ligands of the library have an affinity for a target receptor as measured by a pre-selected indicator compound having known affinity for the target receptor. Using this embodiment, compound libraries can be rapidly screened to identify those libraries that contain ligands capable of shifting the break through time of the pre-selected indicator compound. This shift in break through time for the indicator compound, which can be either a positive time shift or a negative time shift, indicates that putative ligands in the library have an affinity for the target receptor. Optionally, more than one indicator compound can be used for each target receptor or target receptor active site.

Accordingly, in another of its method aspects, this invention provides a method for screening a compound library to determine the relative affinity of a plurality of putative ligands to a target receptor or a plurality of target receptors relative to an indicator compound or a plurality of indicator compounds, which method comprises:

(a) providing a compound library comprising a plurality of putative ligands;

(b) providing at least one void marker compound;

(c) providing an indicator compound or a plurality of indicator compounds for each target receptor, each indicator compound having a pre-determined affinity for the target receptor and having a pre-determined break through time on the column in the absence of the compound library relative to a void marker compound;

(d) applying the compound library to a column comprising a target receptor or a plurality of target receptors, each target receptor optionally bound to a solid phase support, under frontal chromatography conditions to equilibrate or partially equilibrate the column with the compound library;

(e) applying (i) a mixture comprising the compound library, the void marker compound and the indicator compound or compounds, or (ii) the void marker compound and the indicator compound or compounds, to the column under frontal chromatography to provide an effluent;

(f) analyzing the effluent to determine a break through time for the indicator compound or compounds.

In a preferred embodiment, the above method further comprises the step of:

(g) determining whether any putative ligands of the compound library have an affinity for the target receptor as measured by the indicator compound or compounds by comparing the break through time for the indicator compound or compounds from step (f) with the pre-determined break through time for the indicator compound or compounds in the absence of the compound library.

Preferably, the analysis of the effluent in the above method is conducted using a mass spectrometer.

Preferably, when an indicator compound is used, the compound library comprises less than about 50,000 putative ligands, more preferably less than 10,000 putative ligands. Still more preferably, the compound library comprises about 5 to about 100 putative ligands.

Preferably, the indicator compound employed in the methods of this invention has a pre-determined break through time in the absence of the compound library of less than about 5 minutes.

A plurality of columns may also be employed when an indicator compound is used. Accordingly, in still another of its method aspects, the present invention provides a method for screening a plurality of compound libraries to determine the relative affinity of a plurality of putative ligands to a target receptor or a plurality of target receptors relative to an indicator compound, which method comprises:

(a) providing a plurality of compound libraries comprising a plurality of putative ligands;

(b) providing at least one void marker compound;

(c) providing an indicator compound or a plurality of indicator compounds for each target receptor, each indicator compound having a pre-determined affinity for the target receptor and having a pre-determined break through time on each column in the absence of the compound library relative to a void marker compound;

(d) applying each compound library to a separate column comprising a target receptor or a plurality of target receptors, each target receptor optionally bound to a solid phase support, under frontal chromatography conditions to equilibrate or partially equilibrate the column with the compound library;

(e) applying (i) a mixture comprising the compound library, the void marker and the indicator compound or compounds, or (ii) the void marker and the indicator compound or compounds, to each column under frontal chromatography conditions to provide an effluent;

(f) analyzing the effluent from each column to determine a break through time for the indicator compound or compounds.

In a preferred embodiment, the above method further comprises the step of:

(g) determining whether any putative ligands of a compound library have an affinity for the target receptor as measured by the indicator compound or compounds by comparing the break through time for the indicator compound or compounds from step (f) with the pre-determined break through time for the indicator compound or compounds in the absence of the compound library.

An indicator compound can also be used to determine if the overall affinity of a compound library for a target receptor is due to a plurality of weak binders or to one or more strong binders present in the compound library. Accordingly, in yet another of its method aspects, the present invention provides a method for screening a compound library to determine the relative affinity of a plurality of putative ligands to a target receptor relative to an indicator compound having a pre-determined affinity for the target receptor, which method comprises:

(a) providing a compound library comprising a plurality of putative ligands;

(b) providing at least one void marker compound;

(c) providing a column comprising a target receptor optionally bound to a solid phase support;

(d) providing an indicator compound having a pre-determined affinity for the target receptor and having a pre-determined break through time on the column in the absence of the compound library relative to a void marker compound and having a pre-determined signal intensity in the presence of the compound library;

(e) applying a mixture comprising the compound library and the indicator compound to the column under frontal chromatography conditions to provide an effluent;

(f) analyzing the effluent to determine a break through time and/or signal intensity for the indicator compound.

In a preferred embodiment, the above method further comprises the step of:

(g) determining whether any putative ligands of the compound library have an affinity for a target receptor as measured by the indicator compound by comparing the break through time for the indicator compound from step (f) with the predetermined break through time for the indicator compound in the absence of the compound library.

In another preferred embodiment, the above method further comprises the step of:

(h) determining whether the affinity for the target receptor is due to a plurality of ligands having weaker affinity for the target receptor relative to the indicator compound or to one or more ligands having stronger affinity for the target receptor relative to the indicator compound by comparing the signal intensity of the indicator in the effluent with the pre-determined signal intensity for the indicator.

The methods of the present invention also allow the loss of function of a target receptor, such as an enzyme, to be monitored in the presence of a compound-library. Using this embodiment, compound libraries containing inhibitors of a target receptor can be readily identified. Accordingly, in another of its method aspects, this invention provides a method for screening a compound library to identify inhibitors of a target receptor, which method comprises:

(a) providing a compound library comprising a plurality of putative ligands,

(b) applying the compound library to a column comprising a target receptor optionally bound to a solid phase support under frontal chromatography conditions;

(c) providing a first indicator compound which is capable of being chemically modified by the target receptor to form a second indicator compound;

(d) applying (i) a mixture comprising the compound library and the first indicator compound, or (ii) the first indicator compound, to the column under frontal chromatography conditions to provide an effluent;

(e) analyzing the effluent to determine the presence and/or concentration of the first indicator compound and/or the second indicator compound.

Preferably, the target receptor employed in the above method is a catalyst and the first indicator compound is a reactant for the catalyst. More preferably, the target receptor is an enzyme and the first indicator compound is a substrate for the enzyme.

The methods of this invention can also be employed to screen a target receptor or a plurality of target receptors for affinity to an immobilized ligand or plurality ligands. This embodiment is particularly useful for target validation studies on ligands having biological effects. Accordingly, in another of its method aspects, this invention provides a method for screening a target receptor or a plurality of target receptors to determine the relative affinity of the receptor or receptors to an immobilized ligand or ligands relative to an indicator compound or a plurality of indicator compounds, which method comprises:

(a) providing a target receptor or a plurality of target receptors;

(b) providing a column comprising a ligand or a plurality of ligands, each ligand bound to a solid phase support;

(c) providing at least one void marker compound;

(d) providing an indicator compound or a plurality of indicator compounds for each ligand, each indicator compound having a pre-determined affinity for the ligand and having a pre-determined break through time on the column relative to a void marker compound;

(e) applying the target receptor or receptors to the column under frontal chromatography conditions to equilibrate or partially equilibrate the column with the target receptor or receptors;

(f) applying (i) a mixture comprising the target receptor or receptors, the void marker compound and the indicator compound or compounds, or (ii) the void marker compound and the indicator compound or compounds, to the column under frontal chromatography conditions to provide an effluent;

(g) analyzing the effluent to determine a break through time for the indicator compound or compounds.

In a preferred embodiment, the above method further comprises the step of:

(h) determining whether any of the target receptors has an affinity for the ligand or ligands as measured by the indicator compound or compounds by comparing the break through time for the indicator compound or compounds from step (g) with the pre-determined break through time for the indicator compound or compounds.

This method can also be used to screen a compound libraries comprising a plurality of putative ligands immobilized on a solid phase support for affinity to a target receptor or a plurality of target receptors.

In the methods of this invention, the effluent from the column can be introduced directly into the mass spectrometer or the effluent can be collected in fractions and the fractions subsequently analyzed by mass spectrometry. In either embodiment, the effluent can optionally be pre-treated, such as by desalting or by derivatizing the components, prior to analysis. Accordingly, in one preferred embodiment, each of the above methods optionally comprise the steps of collecting the effluent from column to provide a plurality of effluent fractions; and optionally desalting each effluent fraction prior to mass spectral analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a representative apparatus for screening compound libraries using frontal chromatography in combination with a mass spectrometer.

FIG. 2 illustrates a representative apparatus for screening compound libraries using a plurality of frontal chromatography columns in combination with a mass spectrometer.

FIG. 3 illustrates another representative apparatus for screening compound libraries using a plurality of frontal chromatography columns in combination with a mass spectrometer.

FIG. 4 illustrates a representative apparatus for sequentially screening compound libraries with an indicator compound using a plurality of frontal chromatography columns in combination with a mass spectrometer.

FIG. 5A shows a total ion chromatogram (TIC) from a FC-MS run using six representative oligosaccharides having varying affinity for a carbohydrate-binding antibody that recognizes the 3,6-dideoxy-D-galactose (abequose) epitope in Salmonella paratyphi B O-antigens.

FIG. 5B shows selected ion chromatograms for the six oligosaccharides reconstructed from the TIC shown in FIG. 5A.

FIGS. 5C, 5D and 5E show mass spectra generated from time-slices of the TIC shown in FIG. 5A.

FIG. 6 shows a plot of ([A]₀(V−V₀))⁻¹ versus [A]₀ ⁻¹ for αGal(1→2)[αAbe(1→3)]αMan-OCH₃.

FIG. 7A shows a selected ion chromatogram from a FC-MS run using an indicator compound in the absence of a compound library.

FIG. 7B shows a selected ion chromatogram from a FC-MS run using an indicator compound in the presence of a compound library.

FIG. 8 shows a selected ion chromatogram from a FC-MS run using four representative oligosaccharides having varying affinity for cholera toxin B subunit.

FIG. 9 shows a selected ion chromatogram from a FC-MS run using a synthetically prepared GM₁ analog.

FIG. 10 illustrates the “roll-up” effect in a selected ion chromatogram from a FC-MS run using an indicator compound in the presence of a compound library.

FIG. 11 is a graph of the reduction of column activity as a function of time for two different compound libraries, the first library containing many weak binders and the second library containing strong binders.

FIG. 12A shows schematically the synthesis of a compound library containing 100 tripeptides. FIG. 12B shows an electrospray mass spectrum of the compound library.

FIG. 13 shows a chromatogram of three infusion/wash cycles of a compound library containing 100 tripeptides, an indicator compound (dashed line) and a void marker compound (solid line).

FIG. 14A illustrates the V−V₀ value for an indicator compound (dashed line) relative to a void marker compound (solid line) immediately before equilibration of a column with a compound library containing 100 tripeptides. FIG. 14B illustrates the V−V₀ value for the indicator compound immediately after equilibration of the column with the compound library.

FIG. 15A shows a total ion chromatogram for a compound library containing 100 tripeptides. FIG. 15B shows a selected ion chromatogram for m/z 419.2.

FIG. 16A shows a selected ion chromatogram for fPR. FIG. 16B shows a selected ion chromatogram of fPR and fPR-chloromethyl ketone.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for screening compound libraries using frontal chromatography in combination with mass spectrometry. When describing the methods of this invention, the following terms have the following meanings, unless otherwise indicated. All terms not defined herein have their conventional art-recognized meaning.

Definitions

The term “buffer” refers to a solution that stabilizes the binding activity of the target receptor. Typical buffers include, by way of illustration, pH buffers and buffers containing specific molecules, either organic or inorganic, required to stabilize the binding activity of a specific target receptor.

The term “break through time” refers to the period of time between elution of the void volume and the front corresponding to the elution of a particular compound during frontal chromatography. The term “break through curve” refers to the signal intensity as a function of time resulting from the infusion of compound(s) through a column under frontal chromatography conditions. Typically, the break through curve is comprised of a front (fast-rising section) and a plateau (horizontal section).

The term “compound library” refers to a mixture or collection of one or more putative ligands generated or obtained in any manner. Preferably, the library contains more than one putative ligand or member.

The term “electrospray” refers to the generation of gas-phase ions from a flowing solution. Electrospray is typically performed at atmospheric pressure in an electric field with or without assisted nebulization and solvent evaporation.

The term “effluent” refers to the solvent or solution emerging or exiting from the frontal chromatography column.

The term “frontal chromatography conditions” refers to chromatography conditions in which a solution of compounds, such as putative ligands and/or indicator compounds, is applied or infused through a column to generate a break through curve. Typically, under frontal chromatography conditions, putative ligands are infused continuously at a constant concentration through a column containing a target receptor such that the target receptor is continuously contacted with the putative ligands during the chromatography.

The term “indicator compound” or “indicator” refers to a compound having a known affinity or specificity for the target receptor and a measurable break through time under frontal chromatography conditions. When screening target receptors for affinity to a particular ligand(s), the indicator may also be a compound, such as a protein, or other biological entity, such as a cell or cell fragment, having a known affinity or specificity for the ligand(s). The break through time for the indicator is typically referenced to the void volume of the system.

The term “ligand” refers to a molecule or group of molecules that bind to one or more specific sites of a receptor. Representative ligands include, by way of illustration, carbohydrates, monosaccharides, oligosaccharides, polysaccharides, amino acids, peptides, oligopeptides, polypeptides, proteins, nucleosides, nucleotides, oligonucleotides, polynucleotides, including DNA and DNA fragments, RNA and RNA fragments and the like, lipids, retinoids, steroids, glycopeptides, glycoproteins, glycolipids, proteoglycans and the like, and synthetic analogues or derivatives thereof, including peptidomimetics, natural products or naturally-occurring small molecule organic compounds (i.e., compounds produced by and/or isolated from natural sources, such as soil, water, cells, plants, fungi, animals and the like), synthetic small molecule organic compounds, inorganic ions, organometallic compounds and the like, and mixtures thereof. The term “putative ligand” refers to a ligand whose affinity or specificity for a target receptor, if any, has not been determined.

The term “microcolumn” refers to a column having an internal diameter less than or equal to about 1 mm.

The term “natural products” refers to compounds isolated from natural sources, such as cells, plants, fungi, animals and the like. The term “naturally-occurring small molecule organic compounds” refers to natural products that are organic compounds generally having a molecular weight less than about 1000, preferably less than about 500.

The term “selected ion chromatogram” refers to a plot of ion abundance vs. time constructed from the intensity of a single ion. A selected ion chromatogram can be prepared from a scan or selected ion monitoring mode.

The term “selected ion monitoring” refers to the detection of a few pre-selected ions using a mass spectrometer (e.g. quadrupoles).

The term “signal intensity” refers to the measured response of an instrument to a stimulus, for example, the current output of a high energy dynode detector resulting from the impact of an ion.

The term “solid support” or “solid phase support” refers to an inert material or molecule to which a target receptor may be bound or coupled, either directly or through a linking arm.

The term “synthetic small molecule organic compounds” refers to organic compounds generally having a molecular weight less than about 1000, preferably less than about 500, which are prepared by synthetic organic techniques, such as by combinatorial chemistry techniques.

The term “supplemental diluent” or “make-up flow” refers to a solution or solvent which is combined with the effluent from a column before the effluent passes through the mass analyzer of a mass spectrometer.

The term “target receptor” or “receptor” refers to a molecule or a group of molecules capable of binding a ligand at a specific site. Representative examples of target receptors include, by way of example,proteins, including recombinant proteins, glycoproteins, glycosaminoglycans, proteoglycans, integrins, enzymes, lectins, selecting, cell-adhesion molecules, toxins, bacterial pili, transport proteins, receptors involved in signal transduction or hormone-binding, hormones, antibodies, major histocompatability complexes, immunoglobulin superfamilies, cadherins, DNA or DNA fragments, RNA and RNA fragments, whole cells, cell fragments, tissues, bacteria, fungi, viruses, parasites, preons, and synthetic analogs or derivatives thereof.

The term “target receptor active site” refers to the binding site of interest on a particular target receptor.

The term “total ion chromatogram” refers to a plot of ion abundance vs. time constructed from a summation of all ion intensities in a scan. In a total ion chromatogram, the number of scans are linearly related to time.

The term “void marker compound” or “void marker” refers to a compound that elutes from column at the void volume. The void marker compound is used to identify the void volume of a column used under frontal chromatography conditions.

The term “void volume” or “V₀” refers to the volume of solution which passes through a frontal chromatography column from the point of infusion to the point of detection of a compound, i.e. a putative ligand, in the absence (or suppression) of a binding event. Since the flow rate is typically constant, void volume is generally specified in terms of a retention time for the compound. Putative ligands having no affinity for the target receptor typically elute from column at the void volume.

The compound libraries employed in this invention may be prepared or obtained by any means including, but not limited to, combinatorial chemistry techniques, fermentation methods, plant and cellular extraction procedures and the like. Methods for making combinatorial libraries are well-known in the art. See, for example, E. R. Felder, Chimia 1994, 48, 512-541; Gallop et al., J. Med. Chem. 1994, 37, 1233-1251; R. A. Houghten, Trends Genet. 1993, 9, 235-239; Houghten et al., Nature 1991, 354, 84-86; Lam et al., Nature 1991, 354, 82-84; Carell et al., Chem. Biol. 1995, 3, 171-183; Madden et al., Perspectives in Drug Discovery and Design 2, 269-282; Cwirla et al., Biochemistry 1990, 87, 6378-6382; Brenner et al., Proc. Natl. Acad. Sci. USA 1992, 89, 5381-5383; Gordon et al., J. Med. Chem. 1994, 37, 1385-1401; Leb1 et al., Biopolymers 1995, 37 177-198; and references cited therein. Each of these references is incorporated herein by reference in its entirety.

Any type of molecule that is capable of binding to a target receptor may be present in the compound library. For example, compound libraries screened using this invention may contain naturally-occurring molecules, such as carbohydrates, monosaccharides, oligosaccharides, polysaccharides, amino acids, peptides, oligopeptides, polypeptides, proteins, nucleosides, nucleotides, oligonucleotides, polynucleotides, including DNA and DNA fragments, RNA and RNA fragments and the like, lipids, retinoids, steroids, glycopeptides, glycoproteins, glycolipids, proteoglycans and the like; or analogs or derivatives of naturally-occurring molecules, such peptidomimetics and the like; and non-naturally occurring molecules, such as “small molecule” organic compounds generated, for example, using combinatorial chemistry techniques; organometallic compounds, inorganic ions, and mixtures thereof. The term “small molecule organic compound” refers to organic compounds generally having a molecular weight less than about 1000, preferably less than about 500.

A particular advantage of the present method is that compound libraries containing isomers may be screened to determine, for example, if only one isomer (e.g. an enantiomer or diastereomer) is binding to the target receptor, or if the isomers have different affinities for the target receptor. In this regard, if the isomers have different affinities for the target receptor, a different break through time will be observed for each isomer.

The compound libraries employed in this invention will typically contain a plurality of members or putative ligands. When an indicator compound is employed, the compound library will preferably contain less than about 50,000 members, more preferably, the compound library will contain less than about 10,000 members. When an indicator compound is not employed, the compound library will preferably contain less than about 10,000 members; more preferably, from 1 to about 1,000 members; and still more preferably, from about 5 to about 100 members.

The present method is useful for analyzing the affinity of members of a compound library for any target receptor or domain which binds or complexes with a ligand. For example, the target receptor may be selected from, but is not limited to, proteins, including recombinant proteins, glycoproteins, glycosaminoglycans, proteoglycans, integrins, enzymes, lectins, selecting, cell-adhesion molecules, toxins, bacterial pili, transport proteins, receptors involved in signal transduction or hormone-binding, hormones, antibodies, major histocompatability complexes, immunoglobulin superfamilies, cadherins, DNA or DNA fragments, RNA and RNA fragments, whole cells, cell fragments, tissues, bacteria, fungi, viruses, parasites, preons, and synthetic analogs or derivatives of any of the above. If desired, more than one target receptor may be employed when screening a compound library using the methods of this invention.

When employing the methods of this invention, the target receptor (or a ligand) is optionally bound or coupled to a solid support. Preferably, the target receptor is covalently bound or coupled to the solid support. However, in some cases, such as when whole cells or organisms are employed as the target receptor, the cells or organisms may be contained within the column by using, for example, a porous frit or membrane at the outflow end of the column. Supports for receptors and libraries are well-known in the art and many are commercially available. Any such conventional support may be used in this invention. Representative supports include, by way of illustration, polymeric (resin) beads, polymeric gels, glass beads, silica chips and capillaries, agarose, diatomaceous earths, pulp, and the like. When silica capillaries are used as the solid support, the target receptor is bound-directly to the walls of the column. Preferred solid supports for use in this invention are those having minimal non-specific binding properties. A preferred solid support is derivatized porous polystyrene-divinylbenzene polymer beads, such as POROS beads (available from Perseptive Biosystems, Framingham, Mass.). A particularly preferred solid support is silica particles, such as controlled pore glass (available from CPG Inc., Lincoln Park, N.J.).

The target receptor (or a ligand) can be bound or coupled to the support using any art-recognized procedure. For example, the target receptor can be bound using direct immobilization techniques (i.e., covalent binding via a sulfhydryl, amino or carboxyl group and the like), covalent binding through a linking or spacer arm, biotin-avidin binding, biotin-streptavidin binding, antibody binding such as antibody-protein A binding, GST-glutathione binding, ion exchange absorption, hydrophobic interaction, expression of the target receptor as a recombinant protein fused to maltose binding protein, fusion of the target receptor with a peptide which binds selectively to an affinity column, genetically engineered proteins with, for example, His tags or biotin, and the like. Such methods are well-known in the art and kits for practicing many of these methods are commercially available. See, for example, Stammers et al . . . , FEBS Lett. 1991, 283, 298-302; Herman et al . . . , Anal. Biochemistry 1986, 156, 48; Smith et al., FEBS Lett. 1987, 215, 305; Kilmartin et al., J. Cell. Biol. 1982, 93, 576-582; Skinner et al., J. Biol. Chem. 1991, 266, 14163-14166; Hopp et al., Bio/Technology 1988, 6, 1204-1210; H. M. Sassenfeld, TIBTECH 1990, 8, 88-93; Hanke et al., J. General Virology 1992, 73, 654-660; Ellison et al., J. Biol. Chem. 1991, 267, 21150-21157; U. K. Pati, Gene 1992, 114, 285-288; Wadzinski et al., J. Biol Chem. 1992, 267, 16883-16888; Field et al., Mol. Cell. Biol. 1988, 8, 2159-2165; Gerard et al., Biochemistry 1990, 29, 9274-9281; Ausselbergs et al., Fibrinolysis 1993, 7, 1-13; Hopp et al., Biotechnology 1988, 6, 1205-1210; Blanar et al., Science 1992, 256, 1014-1018; Lin et al., J. Org. Chem. 1991, 56, 6850-6856; Zastrow et al., J. Biol. Chem. 1992, 267, 3530-3538; Lim et al., J. Infectious Disease 1990, 162, 1263-1269; Goldstein et al., Virology 1992, 190, 889-893; and the articles in IBI FLAG Epitope Vol. 1: No. 1, September 1992; and references cited therein. Each of these references is incorporated herein by reference in its entirety.

In a preferred embodiment of this invention, the target receptor is bound or coupled to the solid support using biotin-avidin, biotin-streptavidin or a related-type binding. In this procedure, the target receptor is typically biotinylated with a biotin reagent containing a spacer arm. The biotinylated target receptor is then contacted with an avidin-containing solid support. The resulting biotin-avidin complex binds the target receptor to the solid support.

Procedures for biotinylating biomolecules are well-known in the art and various biotin reagents are commercially available. See, for example, E. A. Bayer et al., Meth. Enzymol. 1990, 184, 51; U. Bickel et al., Bioconj. Chem. 1995, 6, 211; H. Hagiwara et al., J. Chromatog. 1992, 597, 331; “Avidin-Biotin Chemistry Handbook” (available from Pierce, Rockford, Ill., Catalog Item No.15055) and references cited therein. A preferred biotin reagent is NHS-LC-biotin (available from Pierce). The extent of biotin incorporation using such reagents can be monitored by, for example, matrix-assisted laser desorption/ionization as described in D. C. Schriemer and L. Li, Anal. Chem. 1996, 68, 3382-3387, or by other art-recognized methods as described in the “Avidin-Biotin Chemistry Handbook” (Pierce). Preferably, an average of about 1 to about 50 biotins are incorporated per target receptor, more preferably about 1 to about 10 biotins per target receptor.

The biotinylated target receptor is typically coupled with an avidin- or streptavidin-containing solid support or related material. Such supports are commercially available or can be prepared by art-recognized procedures. Preferred avidin-containing supports include Ultralink-immobilized avidin (available from Pierce) and POROS 20 immobilized streptavidin (available from Perseptive Biosystems). The biotinylated target receptor is typically coupled with the avidin-containing support by contacting the receptor with the support in a suitable buffer, such as phosphate buffered saline (pH 7), for about 0.5 to 4 hours at a temperature ranging from about 4° C. to about 37° C. Preferably, after coupling the biotinylated target receptor to the avidin-containing support, any remaining avidin binding sites on the support are blocked by contacting the solid support with an excess of free biotin.

The target receptor may be bound or coupled to the solid support either prior to or after introducing the solid support material into a column. For example, the biotinylated target receptor may be contacted or incubated with the avidin- or streptavidin-containing solid support and the resulting solid support containing the target receptor subsequently introduced into a column. Alternatively, the avidin- or streptavidin-containing solid support can be first introduced into the column and the biotinylated target receptor then cycled through the column to form the solid support containing the target receptor in the column. Either of these methods may also be used with any of the other previously mentioned procedures for coupling the target receptor to the solid support.

When more than one target receptor is employed, each target receptor can be bound to the same solid support using the procedures described herein. Alternatively, each target receptor can be bound to a separate solid support and the solid support materials containing the target receptors subsequently homogenized and packed into the column.

The solid support material may be introduced into the column using any conventional procedure. Typically, the solid support is slurried in a suitable diluent and the resulting slurry is pressure packed or pumped into the column. Suitable diluents include, by way of example, buffers such as phosphate buffered saline (PBS) solutions, preferably containing a preservative such as sodium azide, and the like.

Generally, the activity of the target receptor will determine the size of the column employed in this invention, i.e., a smaller column volume may be employed when the target receptor has more activity per unit column volume. Typically, the column employed in this invention will have an internal diameter (i.d.) ranging from about 10 μm to about 4.6 mm. Preferably, the internal diameter of the column will be in the range of from about 100 μm to about 250 μm. The column will typically range in length from about 1 cm to about 30 cm, preferably from about 2 cm to about 20 cm. Preferably, the column will have from about 1 fmol to about 10 nmol of target receptor active sites per column; more preferably, from about 0.1 pmol to about 10 nmol of target receptor active sites per column; still more preferably, from about 0.1 pmol to about 100 pmol of target receptor active sites per column.

If an indicator compound is employed, the length of the column and its i.d. will also depend upon the K_(d) of the indicator compound (i.e., a smaller column may be used when the indicator has a higher affinity for the target receptor). Preferably, when an indicator is employed, the column length and i.d. are selected so that the indicator compound elutes a measurable quantity after the void volume.

The body of the column employed in this invention may be comprised of any conventional column body material including, by way of illustration, poly(ether ether ketone) (PEEK), fused silica, silicon microchips, stainless steel, nylon, polyethylene, polytetrafluoroethylene (Teflon) and the like. Preferably, the column body is comprised of poly(ether ether ketone).

Alternatively, the column may be open-faced, such as in thin-layer chromatography (TLC) plate configuration or a gel plate. In this embodiment, the effluent can be analyzed using the electrospray techniques described herein. Alternatively, the plate or plate-like configuration can be subjected to matrix-assisted laser desorption/ionization (MALDI) analysis at any stage of the chromatography to provide a molecular weight analysis for a plurality of positions on the plate.

After the solid support containing the target receptor is introduced or formed in the column, the column is typically flushed with a suitable diluent to remove any unbound target receptor or impurities. Suitable diluents for flushing the column include, for example, phosphate buffered saline, TRIS buffers and the like. If desired, a detergent may also be included in the buffer to facilitate removal of unbound target receptor or impurities.

After the column is flushed, the column is typically equilibrated with a buffer suitable for frontal chromatography and compatible with mass spectrometry. Volatile buffers are generally preferred for use with mass- spectrometry. For frontal chromatography, a buffer is typically selected to promote-receptor-ligand interaction. Suitable buffers for use in FC-MS include, by way of example, ammonium acetate, ammonium formate and the like.

Following equilibration of the column, the compound library is then applied to the column under frontal chromatography conditions. Typically, when applied to the column, the compound library comprises a solution of the library members or putative ligands in a suitable diluent. Typically, the diluent is the buffer solution used to equilibrate the column. Generally, the concentration of the library members in the diluent will range from about 1 μm to about 50 μm. Preferably, the concentration of library members ranges from about 1 nM to about 10 μm.

Procedures for conducting frontal chromatography are well-known in the art. See, for example, K. -I. Kasai et al., Journal of Chromatography 1986, 376, 3347; D. S. Hage et al., Journal of Chromatography B, 1997, 669, 449-525 and references cited therein. The disclosures of these references are incorporated herein by reference in their entirety. Typically, the compound library is continuously applied or infused into the column containing the target receptor. Under these conditions, the target receptor is continuously contacted or challenged with each of the members of the compound library. The column is driven to dynamic equilibrium by continuously applying the compound library to the column. Library members having different binding constants to the target receptor display different break through times or hold-up volumes on the column, i.e., those members having a higher affinity for the target ligand have a longer break through time on the column or a larger hold-up volume until they begin to elute from or break-though the column at their initial infusion concentration. Unlike zonal chromatographic methods, no physical separation of the library members is achieved using frontal chromatography.

During the frontal chromatography, the column is typically at a temperature in range from about 0° C. to about 90° C.; preferably from about 4° C. to about 60° C.; more preferably from about 20° C. to about 40° C.

When a ligand has a very slow on-rate, it may be desirable to conduct the column equilibrium over an extended period of time. The column can be equilibrated by infusing the compound library through the column for a period sufficient to allow the column to reach equilibrium. For example, this can be achieved by increasing the equilibration time, i.e., to about 0.25 to 24 hours; or by reducing the flow rate to about 1% to 10% of the usual flow rate. Alternatively, a sequence of stop-flow cycles may also be conducted.

In the methods of this invention, a mass spectrometer is coupled to the column to analyze the effluent. Mass spectrometry is particularly useful in the present invention since it allows for both detection and identification of the library members present in the effluent. In this regard, mass spectrometry allows the eluting members of the library to be identified based on their mass/charge ratio.

Prior to analyzing the effluent from the column by mass spectrometry, the effluent is optionally diluted with a supplemental diluent or “make-up flow” and the combined flow is directed into, for example, the electrospray mass spectrometer. Typically, the supplemental diluent comprises a major amount of an organic solvent and a minor amount of an aqueous buffer. The organic solvent is selected so as to promote a stable and efficient electrospray. Representative organic solvents suitable for use in the supplemental diluent include, by way of example, acetonitrile, methanol, isopropanol and the like. A preferred organic solvent is acetonitrile. Typically, the amount of supplemental diluent employed is adjusted so that the combined flow rate of the effluent and the supplemental diluent is less than about 100 μL/min. Preferably, the combined flow rate entering the mass spectrometer ranges from about 100 nL/min to about 20 μL/min.

The effluent from the column is typically fed continuously into the mass spectrometer for analysis. Alternatively, the effluent may be collected using conventional fraction collection techniques and then some or all of the effluent fractions introduced subsequently into the mass spectrometer for analysis. If desired, the effluent fractions can be pre-treated, i.e., de-salted or derivatized using conventional techniques, prior to mass spectral analysis.

Methods for analyzing effluents using mass spectrometry are well-known in the art. Any type of mass spectrometry which is capable of directly or indirectly analyzing the components present in a solution may be employed in this invention including, for example, electrospray mass spectrometry (ES-MS), atmospheric pressure chemical ionization (APCI), membrane introduction mass spectrometry (MIMS), continuous flow fast atom bombardment (cf-FAB), thermospray techniques, particle beam, moving belt interfaces and the like. Electrospray mass spectrometry is particularly preferred. Apparatus and techniques for conducting electrospray mass spectrometric analysis are described, for example, in S. J. Gaskell, “Electrospray: Principles and Practice”, J. Mass. Spectrom. 1997, 32, 677-688 and reference cited therein. When the effluent is collected and optionally pre-treated prior to mass spectral analysis, any of the above described ionization methods may be used as well as MALDI, fast atom bombardment (FAB), massive cluster impact, electron impact, chemical ionization, secondary ion mass spec and field desorption ionization techniques.

The mass spectrometer employed in the methods of this invention may be of any type (i.e., scanning or dynamic) including, by way of illustration, quadrupole, time of flight, ion trap, FTICR and the like. Typically, the mass spectrometer parameters are set to provide the highest sensitivity for the eluting compounds. Generally, when an electrospray mass spectrometer is employed, such adjustments will involve optimization of, for example, nebulizer pressure, drying gas flow rate, ion transmission and electrospray needle position. For example, the nebulizer pressure will typically range from about 0 psi to about 60 psi; and the drying gas flow rate will range from about 0 L/min to about 50 L/min. A total ion chromatogram is typically measured and monitored in real-time. The size of the column, the concentration of the compound library and the flow rate will generally determine the run-time. Typical run times range from about 1 min to about 60 min.

Upon completion of the frontal chromatography, the column is optionally regenerated by washing with a large volume of the binding buffer, with or without a competitive ligand. In this regard, a particular advantage of the present method is that denaturing of the target receptor is not required at any point in the procedure. Accordingly, columns may be re-used many times generally with no observable loss of activity or leaching of the target receptor. Alternatively, since the methods of this invention employ very small amounts of target receptor, the column may be disposed of after a single use.

Suitable apparatus for conducting frontal chromatography are described in U.S. patent application Ser. No. 09/069,890, filed on Apr. 29, 1998, now U.S. Pat. No. 6,054,047 and in U.S. Ser. No. 09/276,443, filed on even date herewith and entitled “Apparatus for Screening Compound Libraries,” the disclosures of which are incorporated herein in their entirety. A representative apparatus for conducting the screening methods of this invention is illustrated in FIG. 1. As shown in FIG. 1, a first reservoir 1, containing a buffer solution, and a second reservoir 2, containing a solution of a compound library in a buffer, are connected via tubing 3 to valve 4. In FIG. 1, reservoirs 1 and 2 are syringes although any similar reservoir may be employed. Valve 4 allows the solutions from reservoirs 1 or 2 to be directed into a waste container 5 or into the inflow end of column 6. Column 6 contains the target receptor bound to a solid phase support, the column wall or otherwise retained within the column. The outflow end of column 6 is connected to a mixing tee 7, which is also connected to reservoir 8, containing a supplemental diluent, via tubing 9. The effluent from column 6 is mixed with the supplemental diluent from reservoir 8 in mixing tee 7 and the outflow is directed via tubing 10 to an electrospray mass spectrometer 11. To control the flow from reservoirs 1, 2 and 8, pressure is applied to plungers 12 via, for example, a pump. A simpler manifestation would include just the column 6 connected to the reservoir 2, with the outflow directed via tubing 10 to an electrospray mass spectrometer. Alternatively, a configuration involving an HPLC pump and a valve with an oversized injection loop for the library solution could be used, such an apparatus is described, for example, in E. Breklan et al., Biochem. 1996, 35, 12141-12145.

In another of its embodiments, this invention provides a method for screening a compound library to determine if any member of the library has an affinity for a target receptor that interferes with the binding of a pre-selected indicator compound or a mixture of indicator compounds. In this embodiment, the break through time of an indicator compound having a known affinity for the target receptor is determined after the column has been equilibrated with the compound library and compared to the break through time for the indicator compound in the absence of the compound library. If the indicator compound has a shorter break through time after equilibration with the compound library, the compound library contains one or more ligands having an overall affinity for the target ligand which is higher than the indicator compound. Since an indicator compound can be selected having a relatively short break through time on the column, a significant advantage of this embodiment is that compound libraries can be rapidly screened, e.g., in less than 5 minutes, to identify those libraries having a pre-determined minimum level of affinity for the target receptor. When a library is identified as having the pre-determined minimum level of affinity for the target receptor, the library can be further analyzed using FC-MS to identify the ligands binding to the target receptor.

One advantage of using an indicator compound is that the screening time for each library is significantly reduced since only the indicator compound needs to be monitored relative to a void marker compound. Additionally, since the indicator compound binds to the target receptor at the active site of interest, a change in the break through time for the indicator reflects an affective interaction of a member (or members) of the library with the target receptor. This interaction includes, by way of example, binding at the active site, and binding at a different, non-overlapping site that affects the ability of the target receptor to bind the indicator compound. This method is particularly advantageous in that nonspecific binding to the target receptor that does not alter the active site will not cause a shift in the break through time for the indicator compound. Accordingly, non-specific binding of the library to the target receptor does not provide false leads.

The indicator compound used in this embodiment of the invention is typically selected so as to have a relatively weak affinity for the target receptor. This permits the indicator compound to rapidly elute or break through the column, thus shortening the period of time necessary to monitor the effluent. An indicator compound having a break through time on the column less than about 5 minutes in the absence of the compound library is preferred. Alternatively, an indicator having a strong affinity for the target receptor may be used thereby allowing smaller columns to be employed. When an indicator compound having a strong affinity is used, the compound library will typically be applied to the column at a higher concentration. The break through time for the indicator compound on the column in the absence of the compound library is determined using the FC-MS procedures described herein. The affinity of the indicator compound for the target receptor can be determined using conventional techniques, such as microcalorimetry and the like; or by using the FC-MS methods of this invention. Preferably, the indicator compound will also have a unique mass in comparison to the members of the compound library so that the indicator compound can be unambiguously identified by mass spectrometry. Generally, when using an indicator compound and a quadrupole mass spectrometer, only the m/z of the indicator compound and the compounds representing the void volume are monitored to provide for a greater signal to noise ratio.

Representative examples of indicator compounds suitable for use with specific target receptors include, by way of illustration, αAbe(1→3)αTal-OCH₃ (K_(d)=0.2 mM) for use with a monoclonal antibody that recognizes the 3,6-dideoxy-D-galactose (abequose) epitope in Salmonella paratyphi B O-antigens; phytic acid (K_(d)=1 μM) for use with L-selectin, and the like. Additionally, more than one indicator compound may be employed. The indicator may also be coupled or conjugated to another molecule containing an atom, isotope or molecular fragment which facilitates its detection. For example, the indicator compound can be conjugated to polyethylene glycols (PEGs) so that the mass spectra would contain peaks differing by 44 units thereby facilitating detection of the of indicator compound.

The break through time for the indicator compound is typically measured relative to a void marker compound. The void marker compound is a compound which elutes from the column at the void volume. Preferably, the void marker compound is structurally similar to the indicator compound, but has no affinity for the target receptor of interest. In some cases, putative ligands in the compound library which have no affinity for the target receptor may serve as the void marker compounds.

When a functional target receptor, such as an enzyme, is employed in the methods of this invention, the substrate for the functional target receptor may be used as the indicator. By doing so, the loss of function or inhibition of the target receptor can be monitored in the presence of a compound library. In this embodiment, a first indicator compound is selected which is a substrate for the functional target receptor, i.e. the first indicator is a compound which is capable of being chemically modified by the functional target receptor to produce a second indicator compound. Using the frontal chromatography procedures described herein, the first indicator compound and the compound library to be analyzed are applied to or infused into a column comprising the functional target receptor. The effluent from the column is then monitored for the presence and/or concentration of the first and/or the second indicator compounds, i.e., the substrate and/or the reaction product. An increase in the expected concentration of the first indicator compound or a decrease in the expected concentration of the second indicator compound (as determined by conducting the frontal chromatography of the indicator compounds in the absence of the compound library) indicates that the functional target receptor is being inhibited by one or more members of the compound library. When a compound library is identified as having an inhibitor present in the library for the functional target receptor, the library can be further analyzed using FC-MS to identify the ligands binding to the target receptor.

When using an indicator compound in the methods of this invention, the break through time for the indicator compound is first determined by applying the indicator compound and the void marker compound to the column containing the target receptor under frontal chromatography conditions. The column is then typically equilibrated or partially equilibrated with the compound library to be screened. Generally, the compound library is applied or infused into the column for a time sufficient to allow all of the library members to break through the column. The effluent during this period may be presented to the mass spectrometer for analysis or may be collected for recycling or disposal. Once the column has been equilibrated or partially equilibrated with the compound library, a mixture comprising the compound library, void marker compound and the indicator compound is applied to or infused into the column using the frontal chromatography procedures described herein. Preferably, the indicator compound will be present in the mixture in a concentration less than its K_(d) value. Typically, the indicator compound will be present in an amount ranging from about 1 nM to about 10 μM, more preferably from about 10 nM to about 1 μM. The effluent from the column is analyzed to determine the break through time for the indicator compound in the presence of the compound library and this time period is compared to the pre-determined break through time for the indicator compound to ascertain whether the compound library has an affinity for the target receptor.

Alternatively, the indicator compound and the void marker compound, without the compound library, can be applied or infused into the column after equilibration or partial equilibration of the column with the compound library. This technique allows very strongly bound ligands or those with slow off rates to be detected.

An indicator compound can also be useful in determining whether the overall affinity of a compound library is due to the presence of a plurality of weak binders or to one or more strong binders. In this embodiment, a column equilibration procedure is initiated by infusion of the compound library as described herein. The indicator compound, in the presence of the library and the necessary void markers, is then passed through the column during the initial stages of the equilibrium process (typically at about 1 to about 5 minutes) and a first break through time for the indicator compound is determined. The flow of compound library without the indicator is then reestablished through the column. After a period of time, the indicator/library/void marker solution is again passed through the column and a second break through time is determined. When the same indicator compound is used through this procedure, the intervening time period between each application of the indicator compound can be as short as the time necessary to wash off the indicator compound, and as long as the total equilibration time (e.g., 1 min to 60 minutes, respectively). This cycle can be repeated any number of times. In this method, the discrimination between weak and strong binders occurs because a weak ligand will reach equilibrium on the column sooner than a strong one. Monitoring of the column activity during the equilibration process is illustrated in FIG. 11 which is a graph of the reduction of column activity as a function of time for two different libraries: one containing many weak binders and one containing strong binders. Even though the same overall reduction in column activity is achieved (as measured by the indicator compound), the rate of reduction is slower for the strong ligands compared to the weak ligands.

An alternative method for distinguishing between a plurality of weak binders and one or more strong binders in a compound library is illustrated in FIG. 10. In this embodiment, an indicator compound (V_(n−1) in FIG. 10) is first selected having an affinity for the target receptor which is weaker than the putative ligands of interest (V_(n)) but stronger than those not of interest (V_(1,max)). A mixture comprising the compound library, the void marker compound and the indicator compound at a pre-determined initial concentration and signal intensity is then applied or infused into a column comprising the target receptor under frontal chromatography conditions to provide an effluent. In this embodiment, the column is not pre-equilibrated with the compound library. Preferably, the concentration of the indicator compound in the mixture is greater than or equal to its dissociation constant for the target receptor. The effluent from the column is then analyzed by mass spectrometry to determine a break through time for the indicator compound and its signal intensity. In the presence of a plurality of weak binders (i.e., weaker than the indicator compound), the break through time for the indicator will be less than its break through time in the absence of the compound library and the basic shape of the curve will be unchanged. In the presence of one or more ligands having an affinity for the target receptor greater than the indicator compound, the break through time will also be less than the break through time for the indicator compound in the absence of the compound library, but the shape of the curve will also display a “roll up” effect as illustrated in FIG. 10. This “roll up” effect is due to the removal of bound indicator compound by the stronger ligand(s). Thus, for a short period of time, the concentration of the indicator compound is higher than its infusion concentration until the stronger ligand(s)-breaks through. The amount of bound indicator compound removed is dependent upon the K_(d) value and the concentration of the stronger ligand(s) present in the library. Weaker ligands do not exert this “roll up” effect because they propagate through the column more quickly than the indicator compound. Detection or measurement of this “roll up” or bump in the break through curve (typically measured as a change in signal intensity) indicates the presence of ligands in the compound library having an affinity for the target receptor greater than the indicator compound. The detection or measurement of a “roll up” effect may also be used when screening target receptors for affinity to an immobilized ligand(s).

In addition to detecting the indicator compound using mass spectrometry, other methods of detection may also be employed. Any detection method that can measure the indicator compound over the background signal of the library compounds can be used. For example, an indicator compound can be detected in the effluent from the column using, by way of example, fluorescence, infra-red absorption, UV-visible absorption, nuclear magnetic resonance (NMR), atomic spectroscopy (i.e., atomic adsorption spectroscopy (AAS), inductively coupled plasma-optical emission spectroscopy (ICP-OES), etc.), flow cytometry, electrochemical detection and the like. Procedures and apparatus for detecting compounds using such methods are well-known in the art and any conventional procedures and apparatus may be used.

The methods of this invention allow a plurality of FC-MS analyzes to be conducted simultaneously using a single mass spectrometer to intermittently monitor each column. Unlike “capture and release” methods which typically provide an elution peak or “spike” for each ligand, FC-MS does not require constant effluent monitoring because once a library member breaks through the column, that member is continuously present in the effluent and can be detected by the mass spectrometer. Therefore, a plurality of FC-MS analyses can be conducted simultaneously using a single mass spectrometer to intermittently monitor each column. For example, using the methods of this invention, at least about 100 columns can be conducted simultaneously.

When employing multiple columns, each column is typically monitored for a brief period of time before switching to the next column. For example, with a quadrupole mass spectrometer, each column is typically monitored sequentially for a period of about 0.5 seconds to about 10 seconds, preferably for about 1 second to about 5 seconds, before switching to the next column. The effluent from each column is analyzed as described herein using mass spectrometry. Generally, a single data file is used to collect all of the data from the multiple column thereby generating a composite total ion chromatogram. Subsequently, separate total ion chromatograms for each column are recreated by synchronizing column switching with mass spectrometry data acquisition.

In a preferred embodiment, each column will have an individual electrospray needle for injection of the column's effluent into the electrospray mass spectrometer. Any geometric arrangement of multiple electrospray needles that allows for fast and repetitive sequences of needle advancement may be employed. A suitable apparatus for the injection of multiple effluents into an electrospray mass spectrometer is described in U.S. patent application Ser. No. 09/069,656, filed on even date herewith, and entitled “Device for Delivery of Multiple Liquid Sample Streams to a Mass Spectrometer,” and now U.S. Pat. No. 6,191,418 the disclosures of which are incorporated herein in their entirety. Alternatively, a linear moving row of electrospray needles (sprayers) and the like may be employed. See, for example, Q. Xue et al., Anal. Chem. 1997, 69, 426-430 and references cited therein, the disclosed of which is incorporated herein by reference in its entirety.

A representative apparatus for screening compound libraries using a plurality of columns is illustrated in FIG. 2. As shown in FIG. 2, each of a plurality of columns 13 is connected via tubing 14 and tee 15 to a first reservoir 16, containing a solution of a compound library in a binding buffer, and a second reservoir 17, containing the binding buffer. In FIG. 2, reservoirs 16 and 17 are syringes although any similar reservoir may be employed. Each column 13 contains a target receptor bound to a solid phase support. The buffer solution in reservoir 17 is used to wash column 13 before or after introduction of the compound library. The outflow end of each column 13 is connected to a mixing tee 18, which is also connected to reservoir 19, containing a supplemental diluent, via tubing 20. The effluent from each column 13 is mixed with the supplemental diluent from reservoir 19 in mixing tees 18 and the outflow is directed via tubing 20 and valves 21 into an electrospray mass spectrometer 22, via an electronically-actuated multi-port selection valve 23, or into waste/recovery containers 24. To control the flow from reservoirs 16, 17 and 19, pressure is applied to plungers 25 via, for example, pumps.

Alternatively, in another embodiment illustrated in FIG. 3, the outflow from mixing tees 18 may be directed via tubing 20 into individual electrospray needles 26 for mass spectrometer analysis.

When using an indicator compound, sequential runs of multiple columns may be advantageous since this allows the retention time for the indicator compound to be more accurately determined. Parallel infusion of the indicator through a plurality of columns is feasible provided an apparatus is used with a suitable high sampling rate (e.g., allowing for a minimum of five mass spectral measurements on the break through curve of the indicator for each of the columns. Such an apparatus is described in U.S. patent application Ser. No. 09/069,656, filed Apr. 29, 1998, and U.S. Pat. No. 6,191,418, filed on even date herewith.

A representative apparatus for sequentially screening compound libraries with a indicator compound using a plurality of columns is illustrated in FIGS. 2, 3 and 4. The apparatus shown in FIGS. 2 and 3 are preferred and are employed as described herein for FC/MS. However, reservoir 16 optionally contains a solution of the compound library plus the indicator compound and void marker compounds in a binding buffer, while reservoir 17 optionally contains a solution of only the compound library in the binding buffer. Alternatively, the apparatus illustrated in FIG. 4 can be used. As shown in FIG. 4, a plurality of reservoirs 27 (e.g., syringes) are held in place with clamp 38. Each reservoir 27 contains a mixture of a compound library and an indicator compound in a suitable diluent (or, alternatively, simply the indicator). The end of each reservoir 27 is connected via tubing 29 to the inflow end of a column 30 containing the target receptor bound to a solid phase support. The outflow end of each column 30 is connected via tubing 31 to an electronically-actuated multiport stream selection valve 32 which controls the flow of the effluent from columns 30. Using valve 32, the effluent from the columns may be directed into a waste container 33, via tubing 34, or into mixing tee 35, via tubing 36. Mixing tee 35 is also connected to reservoir 36, containing a supplemental diluent, via tubing 37. The effluent from each column 30 is mixed with the supplemental diluent from reservoir 36 in mixing tee 35 and the outflow is directed via tubing 38 into an electrospray mass spectrometer 39. To control the flow from the reservoirs 27 into columns 30, a stand-off block 40 may be employed. When pressure is applied to stand-off block 40 via, for example, a pump, the plunger 41 of each reservoir 27 is individually depressed in sequence thereby infusing the contents of the reservoir through tubing 29 into the corresponding column 30. The effluent emerging from each column 30 is sequentially directed into mass spectrometer 39 for analysis.

The methods of this invention also permit the absolute affinity or dissociation constant, K_(d), for certain individual members of a compound library to be readily determined. In this regard, ligands having an affinity for the target receptor break through the column at volumes (i.e., break through times) related to their concentrations and K_(d) values, according to the following equation: ${V_{x} - V_{0}} = \frac{B_{t}}{\lbrack X\rbrack_{0} + \left( K_{d} \right)_{x}}$

where B_(t) represents the dynamic binding capacity of the column; [X]₀ is the infusion concentration of the ligand in the compound library; K_(d) is the dissociation constant for the ligand; V₀ is the void volume; and V_(x) represents the volume at the mid-point of the front corresponding to the break through of the ligand. This simple equation indicates that, once B_(t) and the concentration of the ligand are known, the dissociation constant of a ligand can be determined from a single measurement of its V_(x)−V₀. This equation strictly applies only in the case of a single ligand. In many cases, however, this equation or a modification of it can be applied to multiple ligands as well.

In order to determine B_(t), a representative compound, e.g., compound X, is infused through the column at various concentrations and the corresponding V_(x)−V₀ values measured. A plot of ([X](V−V₀))⁻¹ versus [X]⁻¹ is generated, where the y-intercept indicates the dynamic binding capacity of the column (B_(t)) (analogous to a Lineweaver-Burk plot).

Once the dynamic binding capacity of the column has been determined, the dissociation constants for individual members of the compound library can be determined from a single FC-MS run. For example, the K_(d) for compounds where [X]<<(K_(d))_(x) is determined simply from B_(t)/(V−V₀). For those members of the library with a low dissociation constant, knowledge of their concentration or infusion of the compound library at higher dilution is required to determine K_(d).

The following examples are offered to illustrate this invention and are not to be construed in any way as limiting the scope of this invention. Unless otherwise stated, all temperatures are in degrees Celsius.

EXAMPLES

In the examples below, the following abbreviations have the following meanings. If an abbreviation is not defined, it has its generally accepted meaning.

B_(t) = dynamic binding capacity ° C. = degrees Celsius cm = centimeter eq. = equivalents FAB = fast atom bombardment FC = frontal chromatography g = grams K_(d) = dissociation constant L = liter MALDI = matrix-assisted laser desorption/ionization meq. = milliequivalent mg = milligram mL = milliliter mM = millimolar mmol = millimole MS = mass spectrometry m/z = mass charge ratio N = normal PBS = phosphate buffered saline PEEK = poly(ether ether ketone) pmol = picomole TIC = total ion chromatogram μg = micrograms μL = microliter μm = micrometer μM = micromolar V₀ = void volume

Example 1 Screening of an Oligosaccharide Library Using FC-MS

In this example, a compound library containing a mixture of six oligosaccharides was screened using frontal chromatography in combination with an electrospray mass spectrometer to determine the relative affinity of the oligosaccharides for a monoclonal antibody that recognizes the 3,6-dideoxy-D-galactose (abequose) epitope in Salmonella paratyphi B O-antigens.

The compound library consisted of the following six oligosaccharides: αGalNAc(1→3)βGal-OGr (compound 1); αGal(1→3)[αFuc(1→2)]βGal-OGr (compound 2); αMan(1→3)[αMan(1→6)]βpMan-OGr (compound 3); αAbe(1→3)αTal-OCH₃ (compound 4); αGal(1→2)[αAbe(1→3)]αMan-OCH₃ (compound 5); and αGlc(1→4)βGlc(1→4)αGal(1→2)-[αAbe(1→3)]αMan(1→3)αGlc(1→4)βGlc-OCH₃ (compound 6), wherein Gr=O(CH₂)₈CO₂CH₃. Compound 1-3 were obtained using the procedures described in U.S. Pat. No. 4,362,720 to R. U. Lemieux et al., issued Dec. 7, 1987; U.S. Pat. No. 4,137,401 to R. U. Lemieux et al, issued Jan. 30, 1979; and K. J. Kaur et al., “Use of N-Acetylglucosaminyltransferases I and II in the Preparative Synthesis of Oligosaccharides”, Carbohydr. Res. 1991, 210, 145-153; respectively, the disclosures of which are incorporated herein by reference in their entirety. Compounds 4-6 were obtained using the procedures described in D. R. Bundle et al., “Modulation of Antibody Affinity by Synthetic Modifications of the Most Exposed Pyranose Residue of A Trisaccharide Epitope”, Bioorg. Med. Chem. 1994, 2, 1221-1229, the disclosure of which is incorporated herein by reference in its entirety. Compounds 1-3 are known to have no specificity for the antibody. On the other hand, compounds 4-6 contain the minimal requirement for recognition (abequose) and span a range of affinity for the antibody. The K_(d) values for compounds 4-6, as determined by titration microcalorimetry, are shown in Table 1 below.

The monoclonal antibody used in this experiment was produced as described in D. R. Bundle et al, “Molecular Recognition of a Salmonella Trisaccharide Epitope by Monoclonal Antibody Se155.4” Biochem. 1994, 33, 5172-5182. The antibody (0.5 mg) was biotinylated with a biotin reagent containing a long-chain spacer arm (NHS-LC-biotin, Pierce). The extent of biotin incorporation was monitored by matrix-assisted laser desorption/ionization and the reaction was terminated at 14 biotins/IgG (average). The biotinylated antibody was then coupled to a beaded support by incubating the antibody with 25 μL of Ultralink immobilized avidin (Pierce, Cat. No. 53119) in bicarbonate buffer (pH 8.5) for 1 hour. The beads were then thoroughly washed with the bicarbonate buffer. A UV quantitation indicated an immobilization of ˜45 μg antibody/25 μL beads was achieved. The beads were then slurry-packed into a 500 μm i.d. by 11.5 cm poly(ether ether ketone) (PEEK) column body (˜23 μL column volume).

In this experiment, a mixing tee served a dual role as a column end-fitting and mixing chamber for the column eluent and organic make-up flow. The column was then directly connected to an electrospray mass spectrometer (Hewlett-Packard series 1100 MSD, single quadrupole).

For operation in frontal chromatography mode, the column was first flushed with ammonium acetate buffer (NH4OAc, 2 mM, pH 6.7). After flushing, the flow was switched to a second solution containing a mixture of the six oligosaccharides in ammonium acetate buffer, each present at 1 μm. All solutions were infused concurrently with a multi-syringe pump (PHD 200, Harvard Apparatus) at a flow rate of 8 μL/min/syringe (1 cc syringes). A Rheodyne valve (Model 9725) was used for flow switching. The column effluent combined with the make-up flow (10% 2 mM NH₄OAc buffer in acetonitrile) in the tee to provide a flow rate of 16 μL/min into the mass spectrometer.

For the analysis of this mixture, the spectrometer was scanned from m/z 100-1500. Data was collected in scan mode with positive ion detection. A total ion chromatogram (TIC) was constructed from a 50 minute run time as shown in FIG. 5A. This represented the consumption of only 400 pmol of each oligosaccharide. Peaks at specific m/z values were then identified through the analysis of the mass spectra giving rise to the TIC and selected ion chromatograms for all six compounds were reconstructed from the TIC as shown in FIG. 5B. Compounds 1-3 break through the column simultaneously as indicated by the solid line. Mass spectra were then generated from time-slices of the TIC (at times I, II and III) as shown in FIGS. 5C, 5D and 5E. These mass spectra chart the progression of the various oligosaccharides through the column. A spectrum representing the onset of compound 4 is not shown.

As discussed above, ligands having no affinity for the target receptor break through at the void volume (V₀), while compounds having an affinity for the target ligand break through later, at volumes relating to their concentrations and K_(d) values, according to the following equation: ${V_{x} - V_{0}} = \frac{B_{t}}{\lbrack X\rbrack_{0} + \left( K_{d} \right)_{x}}$

where B_(t) represents the dynamic binding capacity of the column; [X]₀ is the infusion concentration of the ligand in the compound library; K_(d) is the dissociation constant for the ligand; V₀ is the void volume; and V_(x) represents the volume at the mid-point of the front corresponding to the break through of the ligand.

In order to determine B_(t), compound 5 was infused through the column at various concentrations and the corresponding V−V₀ values measured. A plot of ([A]₀(V−V₀))⁻¹ versus [A]₀ ⁻¹ was generated, where A is compound 5, as shown in FIG. 6. The y-intercept indicated a B_(t) of 520 pmol. Each antibody molecule contains two binding sites, therefore this corresponds to an active capacity of 260 pmol of protein (representing 93% of the total amount of protein bound). The x-intercept indicated a K_(d) of 11.2 μM for compound 5, which compares favorably with the value determined by microcalorimetry as shown in Table 1.

Knowledge of the column capacity prior to the screening of a mixture allows for the determination of dissociation constants from a single frontal chromatogram. For compounds with [X]<<(K_(d))_(x), the K_(d) can be determined simply from B_(t)/(V−V₀). For example, compound 4 was shown to have a K_(d) of 0.2 mM, as determined from the chromatogram of FIG. 5B. Compounds with low dissociation constants require the knowledge of their concentration and/or the infusion of the mixture at higher dilution for the determination of K_(d). The K_(d) of compound 6, at a 1 μM concentration, was determined from the same chromatogram to be 1.5 μM. The K_(d) values of the compound determined by infusion of the mixtures agree with those obtained from infusions of the individual ligands, as shown in Table 1.

The column was regenerated offline by washing with a large volume of binding buffer. The column used in this example was subjected to over 150 runs with no observable loss of activity or leaching of the antibody.

The results from this experiment are shown in Table 1.

TABLE 1 K_(d) ± s (μM)² Oligosaccharide Micro- FC/MS No. Gr = O(CH₂)₈CO₂CH₃ (MNa)⁺¹ cal³ Ind.⁴ Mix⁵ 1 αGalNAc(1→3)βGal-OGr 576.3 — — 2 αGal(1→3)[αFuc(1→2)]βGalO-Gr 681.3 — — 3 αMan(1→3)[αMan(1→6)]βMan-OGr 697.3 — — 4 αAbe(1→3)αTal-OCH₃ 347.0 190 185 ± 17  178 ± 23  5 αGal(1→2)[αAbe(1→3)]αMan-OCH₃ 509.2 6.3 12.6 ± 1.3  10.2 ± 1.1  6 αGlc(1→4)βGlc(1→4)αGal(1→2)[αAbe 1157.4 0.88 1.79 ± 0.20 1.71 ± 0.16 (1→3)]αMan(1→3)αGlc(1→4)βGlc- OCH₃ ¹Monoisotopic molecular weight of the singly charged sodium adduct. ²Dissociation constant with the corresponding standard deviation. ³Microcalorimetry. ⁴Values determined from infusion of individual ligand, with Compounds 1-3. ⁵Values determined from the infusion of the six-compound mixture.

The results in Table 1 demonstrate that the affinity of various putative ligands in a compound library for a target receptor can be determined relative to other putative ligands in the library; and that the dissociation constant, K_(d), for putative ligands and the target receptor can be determined. The results further demonstrate that there is an acceptable correlation between the literature K_(d) values and those generated by FC-MS procedures.

Example 2 Screening of an Oligosaccharide Library Using FC-MS and an Indicator Compound

In this example, the use of an indicator compound to screen a compound library is demonstrated. The antibody used in this example was the same as that used in Example 1, i.e., a monoclonal antibody that recognizes the 3,6-dideoxy-D-galactose (abequose) epitope in Salmonella paratyphi B O-antigens. The column was also essentially the same as the column in Example 1 and it was prepared and operated as described therein.

In this experiment, three solutions were prepared. Solution A contained the following four oligosaccharide in 2 mM NH₄OAc: αGalNAc(1→3)βGal-OGr (compound 1); αGal(1→3)[αFuc(1→2)]βGal-OGr (compound 2); αMan(1→3)[αMan(1→6)]βMan-OGr (compound 3); αAbe(1→3)αTal-OCH₃ (compound 4), wherein Gr=O(CH₂)₈CO₂CH₃. Solution B contained αGal(1→2)[αAbe(1→3)]αMan-OCH₃ (compound 5) in 2 mM NH₄OAc; and Solution C contained compounds 1-5 in 2 mM NH₄OAc. In all solutions, compounds 1, 2 and 3 were present at 1 μm, compound 4 was present at 0.16 μM, and compound 5 was present at 15 μM. In this example, compound 4 was used as the indicator compound and compound 5 was used to represented a member of a compound library. The remaining compounds were used to determine V_(o).

Solution A containing compounds 1-4 was infused into the column as described in Example 1. A quadrupole mass spectrometer was used to monitor the effluent. The mass spectrometer was operated in selected ion monitoring (SIM) mode, on the (M+Na)⁺ peak of each compound. FIG. 7A shows the selected ion chromatograms generated from an infusion of compounds 1-4 (i.e., Solution A). The breakthrough volume for compound 4 was 3.0±0.1 μL. The column was regenerated by flushing with the binding buffer (i.e., 2 mM NH₄OAc) for about 10 min. at which time essentially all traces of compound 4 were removed.

Using the apparatus of FIG. 1, Solution B (compound 5) and Solution C (compounds 1-5) were loaded into separate syringes. Solution B was infused through the column until dynamic equilibrium for compound 5 was attained. At this point, the flow was switched to the syringe carrying Solution C, and the selected ion chromatograms of FIG. 7B were generated using the quadrupole mass spectrometer. As shown in FIG. 7B, pre-equilibration of the column with compound 5 leads to a measurable shift in the breakthrough volume of the indicator compound 4 (to 1.1±0.3 μl). This is consistent with the fact that compound 5 is a ligand having a K_(d) for the antibody lower than that of the indicator compound 4 (see Table 1 above). Therefore, by simply monitoring the indicator compound, the fact that the representative library contained a compound with a higher affinity for the target receptor was readily apparent.

Note that while the indicator compound (compound 4) in this experiment was added to a solution of the representative library (compound 5), this will not always be necessary. In those situations where the library (Solution B) contains a strongly retained compound (i.e., low K_(d), or off-rate), Solution A can be substituted for Solution C (i.e., the indicator does not need to be mixed with the library).

Example 3 Screening of an Oligosaccharide Library Using FC-MS

In this example, a compound library containing a mixture of four oligosaccharides was screened using frontal chromatography in combination with an electrospray mass spectrometer to determine the, relative affinity of the oligosaccharides for cholera toxin B subunit.

The compound library consisted of the following four oligosaccharides: αGalNAc(1→3)βGal-OGr (compound 1); αGal(1→3)[αFuc(1→2)]βGal-OGr (compound 2); αMan(1→3)[αMan(1→6)]βMan-OGr (compound 3); and GM₁ oligosaccharide (compound 7, wherein Gr=O(CH₂)₈CO₂CH₃. Compound 7, which is the natural ligand for cholera toxin B subunit, was obtained using the procedures described in A. Schön et al., “Thermodynamics of Intersubunit Interactions in Cholera Toxin upon Binding to the Oligosaccharide Portion of Its Cell Surface Receptor, Ganglioside G_(M1)” Biochem. 1989, 28, 5019-5024, the disclosure of which is incorporated herein by reference in its entirety. Cholera toxin B subunit was obtained from LIST Biochemicals, Campbell, Calif.

A column was prepared from a 12 cm section of 0.01″ (250 μm) i.d. PEEK tubing (column volume of about 6 μL). The column was packed with POROS 20 immobilized streptavidin particles (available from Perseptive Biosystems, Framingham, Mass.).

Cholera toxin B subunit (a pentameric protein) was biotinylated to provide about 1-2 biotins/monomer, as measured by MALDI. A dilute solution of this biotinylated protein (4 μM) was infused through the pre-packed column such that the total amount of cholera toxin B subunit bound was approximately 200 pmol after washing (as determined by UV quantitation).

A solution containing compounds 1-3 and 7 was prepared. All compounds were present at 2 μM, in 2 mM NH4OAc (pH 6.9). Using an apparatus similar to that shown in FIG. 1, the column was first equilibrated with the binding buffer (2 mM N₄OAc). The solution containing compounds 1-3 and 7 was then infused through the column at 8 μL/min. The effluent was combined with a typical make-up flow (10% 2 mM NH₄OAc in acetonitrile) and passed into an electrospray single quadrupole mass spectrometer. Data was collected in scan mode, with negative ion detection.

A total ion chromatogram was generated, followed by reconstruction of selected ion chromatograms for each of compounds 1-3 and 7 as shown in FIG. 8. As illustrated in FIG. 8, compounds 1-3 broke through in the void volume of the system (˜4 min×8 L/min=32 μL) while compound 7 (GM₁ oligosaccharide) broke through at ˜300 μL. Thus, GM₁ oligosaccharide (K_(d)≅100 nM) has a stronger affinity for cholera toxin B subunit than compounds 1-3 which have little or no affinity for cholera toxin B subunit.

A second mixture was then prepared in the binding buffer and analyzed by FC-MS in a similar fashion. This mixture contained a synthetically prepared GM₁ analogue, i.e., βGal(1→3)βGalNAc(1→)—OCH₂CH₂O-(←2)αNeu5Ac, (compound 8) in an impure form (i.e. containing unidentified intermediates and reaction byproducts). Compound 8 was prepared by the methods described in P. Füigedi et al, “A Novel Promoter for the Efficient Construction of 1,2-trans Linkages in Glycoside Synthesis, Using Thioglycosides as Glycosyl Donors” Carbohydr. Res. 1986, 149, C9-C12; A. Marra et al., Stereoselective Synthesis of 2-Thioglycosides of N-Acetylneuraminic Acid”, Carbohydr. Res. 1989, 187, 3542; and L. Lay et al., “Synthesis of the Propyl Glycoside of the Trisaccharide α-L-Fucp-(1→2)-β-D-Galp-(1→3)-β-D-GalpNAc. Components of a Tumor Antigen Recognized by the Antibody Mbr1” Helv. Chim. Acta. 1994, 77, 509-514; the disclosures of which are incorporated herein by reference in their entirety. The mixture was infused through the column, and the mass spectrometer was set to operate in selected ion monitoring mode, on negative ions representative of compounds 3 and 8. Selected ion chromatograms were generated for these ions as shown in FIG. 9. FIG. 9 shows that compound 3 broke through in the void volume (m/z 673.2). A more complex pattern was observed for the ions with a mass/charge of 717.2 u. A certain fraction of these ions also broke through in the void volume (˜25%), while the remaining 75% broke through significantly later (at about 11 min). This two-front profile indicates an impurity of equal mass to charge ratio exists at the 25% level, which does not bind to cholera toxin B subunit. Thus, FC-MS is able to ascertain the presence of isobaric, non-binding impurities. Reasonably accurate quantitation of these impurities can also be achieved.

Example 4 Screening of a Compound Library Containing 100 Putative Ligands Against a Human Enzyme

In this example, a compound library containing a mixture of 100 tripeptides was screened against immobilized human α-thrombin using frontal chromatography in combination with an electrospray mass spectrometer. The peptides were synthesized as a mixture by established solid phase techniques and were purchased from Alberta Peptide Institute (Edmonton, Alberta, Canada). This set of peptides all have a common C-terminal amino acid (arginine), while the remaining two positions are random and chosen from a set of 10 amino acids (see FIG. 12). FIG. 12 also displays an electrospray mass spectrum of the mixture. The spectrum was collected from an infusion of a 50 μm solution in 1:1 acetonitrile:ammonium acetate (2 mM, pH 7.2). Assuming equimolar ratios of all peptides, this corresponds to 0.5 μM per peptide. This spectrum highlights a peak at m/z of 419.2. This peak corresponds to two isomeric entries in the library: PfR and fPR, where f refers to D-phenylalanine. The tripeptide fPR has been identified in the literature as an inhibitor possessing a K_(d) value of approximately 1 μM against human α-thrombin. An experiment was conducted to determine if this ligand could be detected when present in the full mixture as screened against a thrombin column.

A column was prepared from a 5 cm section of 0.01″ (250 μm i.d.) PEEK tubing (column volume of about 2.5 μL). The column was packed with POROS 20 immobilized streptavidin particles (available from Perseptive Biosystems, Framinghan, Mass.). Human α-thrombin was purchased from Sigma Chemical Co., and biotinylated with a reagent containing a long-chain spacer arm (sulfo-NHS-LC-biotin, Pierce). The extent of biotin incorporation was less than 5 biotins/thrombin, as monitored by matrix-assisted laser desorption/ionization. A dilute solution of this biotinylated protein (approximately 2 μm) was infused through the pre-packed column in the presence of 0.1% bovine serum albumin (w/v) such that the total amount of immobilized thrombin was approximately 570 pmol (as calculated by a determination of the reduced capacity of the column for free biotin).

The BOC-protected fPR (BOC-fPR, also known as ligand) was purchased from Calbiochem and infused through the column at a concentration of 1 μM in ammonium acetate solution (2 mM, pH 7.2), and at a flow rate of 8 μL/min. FIG. 13 displays a chromatogram reflecting three infusions/wash cycles of this peptide through the column, in the presence of a void marker compound (a non-binding trimannosyl compound). FIG. 13 shows the selected ion chromatograms for each compound. The average V−V₀ value was determined to be 4.66μL. Based on this experiment, and the infusion of this peptide at higher concentrations, the B_(t) value of the column was calculated to be approximately 145 pmol (˜25% active). The BOC-fPR was selected as the indicator compound for the following work, with the trimannosyl compound the corresponding void marker compound.

The library of peptides was then infused through the column at a concentration of 1 μM per peptide for approximately 30 minutes, whereupon the indicator compound and void market compound (in the presence of the library) were infused. The same buffer and flow rate conditions as above were used. FIG. 14 displays the V−V₀ value immediately before (14A) and immediately after (14B) the 30 minute equilibration time. The drop in V−V₀ as a result of the 100 peptides indicates the loss of virtually all of the binding activity of the protein.

To determine the nature of the compounds giving rise to the indicator shift, the peptide library was infused through a fresh, identically prepared column in FC/MS mode. The HP quadrupole electrospray mass spectrometer was set to scan the mass range from m/z 100 to 600 at a scan rate of approximately 1 sec/cycle. The effluent was monitored in real time, and the experiment was stopped at approximately 15 minutes. The results are displayed in FIGS. 15A and 15B. FIG. 15A shows a featureless total ion chromatogram. However, a generation of selected ion chromatograms from the peaks representing the mixture components resulted in the identification of m/z 419.2 as giving rise to the largest V−V₀ shift (as shown in FIG. 15B). Two breakthrough curves are evident in this Figure, indicating the presence of at least two isomers. Based on a knowledge of the mixture composition, this is consistent with the presence of the isomers PfR (non-binding) and fPR (the ligand). A fresh column was constructed through which a solution of just the void marker compound and fPR (1 μM each) was infused. This generated a V−V₀ that was approximately twice the value measured from the mixture. A similar experiment was conducted using PfR instead, which generated no measurable V−V₀. This confirms that fPR is indeed the ligand in the mixture. The combination of a large indicator shift and a smaller than expected V−V₀ for the ligand is consistent with the presence of additional ligands. Thrombin is a serine protease capable of cleaving the C-terminal side of K and R, therefore a large fraction of these peptides serve as substrates for the enzyme. At the infusion concentrations of the experiment, these peptides compete with the binding of fPR. This shows that FC/MS, in conjunction with the indicator analysis, serve as a rapid means of identifying ligands from mixtures.

An additional experiment was conducted with this system to illustrate the use of the roll-up effect in determining the presence of a strong ligand. A thrombin column similar in construct to those mentioned above was prepared, this time containing approximately 50 pmol of active protein. The ligand fPR was selected as an indicator and infused through the column at a concentration of 1 μM to generate the selected ion chromatogram of FIG. 16A. This represents a typical break through curve. The column was regenerated offline with binding buffer, whereupon a solution containing 1 μM of fPR and 1 μM of fPR-chloromethyl ketone (an affinity label with a K_(d) value of approximately 50 nM) was infused. The selected ion chromatogram is shown in FIG. 16B, where the solid line represents fPR and the dashed line fPR-chloromethyl ketone. For clarity, the selected ion chromatogram for the void marker is not displayed. Firstly, there is a time shift in the break through curve for fPR vs. the break through curve of FIG. 16A. Secondly, the y axis of FIG. 16B indicates a maximum intensity of approximately 3 times that of the infusion concentration, followed by a return to the infusion concentration level. This indicates the presence of a stronger ligand in the mixture that causes the release of prebound indicator (this indicator loaded on to the column ahead of the stronger ligand). Note that the peak correlates with the onset of the break through time for the stronger ligand (dashed line). Therefore, monitoring solely the indicator leads to the identification of a mixture containing at least on ligand with a binding constant lower than the indicator.

From the foregoing description, various modifications and changes in the composition and method will occur to those skilled in the art. All such modifications coming within the scope of the appended claims are intended to be included therein. 

What is claimed is:
 1. A method for screening a compound library to determine the relative affinity of a plurality of putative ligands to a target receptor or a plurality of target receptors relative to an indicator compound or a plurality of indicator compounds, which method comprises: (a) providing a compound library comprising a plurality of putative ligands; (b) providing at least one void marker compound; (c) providing an indicator compound or a plurality of indicator compounds for each target receptor, each indicator compound having a pre-determined affinity for the target receptor and having a pre-determined break through time on the column in the absence of the compound library relative to a void marker compound; (d) applying the compound library to a column comprising a target receptor or a plurality of target receptors, each target receptor selected from the group consisting of proteins, peptides, DNA, DNA fragments, RNA and RNA fragments and optionally bound to a solid phase support, under frontal chromatography conditions to partially equilibrate the column with the compound library; (e) applying (i) a mixture comprising the compound library, the void marker compound and the indicator compound or compounds, or (ii) the void marker compound and the indicator compound or compounds, to the column under frontal chromatography conditions to provide an effluent; (f) analyzing the effluent to determine a break through time for the indicator compound or compounds; and (g) determining whether any putative ligands of the compound library have an affinity for a target receptor greater than the indicator compound by comparing the break through time for the indicator compound from step (f) with the predetermined break through time for the indicator compound in the absence of the compound library.
 2. The method of claim 1, wherein the analysis of the effluent is conducted using a mass spectrometer.
 3. The method of claim 2, wherein the mass spectrometer is an electrospray mass spectrometer.
 4. The method of claim 1, wherein the compound library comprises less than about 50,000 putative ligands.
 5. The method of claim 4, wherein the compound library comprises about 5 to about 100 putative ligands.
 6. The method of claim 1, wherein the compound library comprises putative ligands selected from the group consisting of carbohydrates, monosaccharides, oligosaccharides, polysaccharides, amino acids, peptides, oligopeptides, polypeptides, proteins, nucleosides, nucleotides, oligonucleotides, polynucleotides, lipids, retinoids, steroids, glycopeptides, glycoproteins, glycolipids, and proteoglycans.
 7. The method of claim 1, wherein the compound library comprises putative ligands selected from the group consisting of natural products.
 8. The method of claim 1, wherein the compound library comprises putative ligands selected from the group consisting of synthetic small molecule organic compounds.
 9. The method of claim 1, wherein each target receptor is bound to a solid phase support.
 10. The method of claim 1, wherein the column contains from about 1 fmol to about 10 nmol of target receptor active sites.
 11. The method of claim 1, wherein the pre-determined break through time for the indicator compound in the absence of the compound library is less than about 5 minutes. 